1. Field of the Invention
The present invention relates to a container resistant to extremely low temperatures such as about -196.degree. C. which is the temperature of liquid nitrogen and, more particularly, to a plastic container resistant to extremely low temperatures which is suitable for freezing preservation of food, chemicals or physiological saline solutions such as bacteria, enzymes, and blood components such as erythrocytes, blood platelets, and plasma.
2. Description of the Prior Art
Conventional methods for preserving blood include the ACD blood preservation method utilizing an ACD anticoagulant (blood or blood components are preserved in glass bottles or soft vinyl chloride containers holding an ACD solution); the slow freezing method (blood or blood components are preserved in soft vinyl chloride containers at -80.degree. to -85.degree. C.); or the like. However, according to the former method, blood can only be preserved for 21 days from the blood collection due to the blood metabolism during the preservation. According to the latter method, since high concentration glycerol is used as an additive for preventing the adverse effects of freezing preservation, the blood must be rinsed off. Therefore, the rate of recovery of erythrocytes becomes low, and the quality of erythrocytes is degraded in several years of preservation. Accordingly, both these methods are not suitable for preserving blood over a long period of time.
In order to solve these problems, the quick freezing method has been proposed. In the quick freezing method, a physiological saline solution such as blood components is instantaneously frozen at an extremely low temperature of -150.degree. to -200.degree. C. This method requires a container for holding blood which withstands extremely low temperatures, which is resistant to sterilization, and which is easy to handle. However, a soft vinyl chloride container conventionally used for blood preservation cannot withstand extremely low temperatures such as -196.degree. C. of liquid nitrogen and easily cracks under even a small impact after being frozen. Sealing of inlet or outlet ports of containers of a metal such as aluminum and stainless steel is difficult, so that liquid nitrogen frequently penetrates into the containers. These metal containers are non-transparet and do not allow observation of the contents through the container walls before use and are expensive to manufacture.
Fluorine-contained resins and polyimide are known as high polymers which are highly resistant to extremely high and low temperatures. It has been proposed to use, for freezing preservation of blood, a container bag comprising a laminated body of an inner layer of a tetrafluoroethylene-hexafluoropropylene copolymer, and an outer layer of polybromelliteimide. Of these materials, polyimide has a very high melting point and hence excellent heat resistance, and also has high resistance to extremely low temperatures such that it has flexibility at the temperature of liquid helium (4.degree. K.). Although polymide is transparent, it is dark brown in color and may not allow observation through the container wall depending upon the type of the contents. Polyimide is also one of the high polymers which have low thermal conductivity. This leads to a slow freezing speed which adversely affects the recovery rate or quality of the contents, or also requires strict heat-sealing conditions which results in imcomplete sealing or other problems. Polyimide, above all, is highly expensive.
U.S. Pat. No. 3,576,650 discloses a container for use at extremely low temperatures which uses a biaxially oriented polyolefin film. However, upon heat sealing, the sealed part and the surrounding part return to the state before stretching. Therefore, the sealed part must be protected by a metal net or the like in order to eliminate the shocks against the sealed part. Furthermore, general high-density polyethylene (maximum molecular weight of 500,000) is resistant to low temperatures of up to -100.degree. C. and cannot withstand a low temperature of -196.degree. C.